update requirements in README

README.md

@@ -17,11 +17,12 @@ Frama-Clang
        should be included in their dependencies; otherwise, you might need to
        install them as well)
 - Frama-C version 25.x Manganese
-- OCaml version 4.11.1 or higher
+- OCaml version 4.08.1 or higher
   (i.e. the same version than the one that was used to compile Frama-C).
 - The corresponding `camlp5` version
 - for newer OCaml versions which do not include the `Genlex` module,
-  `camlp-streams` (available through `opam`)
+  `camlp-streams` (available through `opam`, it should normally be installed
+  together with `camlp5` if needed)
 
 The front-end can then be compiled with the traditional commands
 ```